(youngest) Silver Plume Granite: light pinkish gray, fine-grained granite. Pikes Peak Granite: pink, coarse-grained granite. Boulder Creek Granite: dark gray, faintly banded granodiorite. Coal Creek Quartzite: light gray quartzite with grains ranging in size from fine sand to boulders, with some interbedded schist. Swandyke Hornblende Gneiss: dark gray to black, strongly banded gneiss. (oldest) Idaho Springs Formation: gray to black schist and gneiss. From a sequence such as this, it is possible to reconstruct some features of Colorado’s early history. The first chapter of which we have a record is the deposition of the Idaho Springs Formation, probably as an accumulation of mud, sand, and limy mud in an ancient sea. Swandyke deposition followed—the sediments were iron-rich, perhaps derived from ancient volcanic materials. The original Coal Creek sediments were sands and gravels, some of them quite coarse and therefore indicative of near-shore deposition. The schist layers suggest that muds must have been deposited also. [Illustration: South of Ouray, Cambrian sandstones of the Sawatch Formation lie almost horizontally across the vertical Precambrian metamorphic rocks. (Jack Rathbone photo)] Together these three formations represent some 40,000 feet of sedimentary layers. Deposition of such a great thickness of mud, sand, and lime must have taken a very long period of time. Details of the geography of the continent during that period have of course been obscured by later events, when these rocks were subjected to repeated uplift, crumpling, folding, various degrees of remelting and recrystallization, and erosion. But the ancient sediments must have been derived from even more ancient highlands, either folded and faulted mountains or volcanoes, and probably were deposited under water in broad expanses of sea that covered portions of the continent. [Illustration: Geologic map of Colorado. Geologic maps show the age of rocks appearing at the surface, disregarding soil cover. A more detailed geologic map of Colorado may be obtained from the U.S. Geological Survey at the Federal Center in Denver.] PRECAMBRIAN PALEOZOIC MESOZOIC CENOZOIC SEDIMENTS CENOZOIC VOLCANICS Yampa River White River Fort Collins South Platte River Glenwood Springs Denver Colorado River Grand Junction Aspen Gunnison River Colorado Springs Gunnison Salida Dolores River Rio Grande Arkansas River La Junta Walsenburg Alamosa Durango